FIX: PL011 UART driver's configuration for RPi4

[ElectroQuanta]

• Page alignment is required for Bao; all RPi4's UARTs (except for UART0) are not page aligned, thus requiring an offset. This must be defined by the platform's user (plat/platform.h).
• UART_CLK must be overridable and defined by the platform's user (plat/platform.h): in RPi4 PL011 run at 48 MHz